Plain English Meaning
The end piece of the fuel hose that goes into the tank opening when the airplane is being refueled.
Full Definition
The metal spouts at the end of a fuel hose that are inserted into an aircraft's fuel tank opening to deliver fuel during refueling. Aviation fuel filler nozzles are sized and sometimes shaped differently for avgas and jet fuel to help prevent misfueling.
Context Anchor
You encounter fuel filler nozzles during refueling, preflight planning, and any discussion of preventing the wrong fuel from being put into an aircraft.
Derivation
“Fuel” comes from an old word meaning material for a fire. “Filler” means something used to fill an empty space. “Nozzle” originally referred to a small projecting nose or spout. Together, the term points to the spout used to fill the airplane with fuel.